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Math Help - bulb problem

  1. #1
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    bulb problem

    A light bulb uses 1.2 A when connected across a 120 V source. What power is used by the bulb?

    Also, what current flows through a 75W bulb connected to a 120 V outlet?

    thanks for any help with this question.
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by rcmango View Post
    A light bulb uses 1.2 A when connected across a 120 V source. What power is used by the bulb?
    the power, P = I^2 * R = (V^2)/R
    but what is R? we can find it using ohms law

    V = IR
    => R = V/I = 120/1.2 = 100

    => P = (V^2)/R = (120^2)/100 = 144 watts


    Also, what current flows through a 75W bulb connected to a 120 V outlet?
    P = (V^2)/R
    => 75 = (120^2)/R
    => R = (120^2)/75 = 192 ohms

    Now V = IR
    => I = V/R = 120/192 = 0.625 amps

    or we could have found I directly, without first finding R:

    P = (I^2)*R
    since V = IR => R = V/I
    so P= (I^2)(V/I) = IV
    => I = P/V = 75/120 = 0.625
    Last edited by Jhevon; May 1st 2007 at 09:02 PM.
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  3. #3
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    thanks for breaking this one down for me.
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by rcmango View Post
    A light bulb uses 1.2 A when connected across a 120 V source. What power is used by the bulb?

    Also, what current flows through a 75W bulb connected to a 120 V outlet?

    thanks for any help with this question.
    P=IV

    RonL
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